1. Field of the Invention
The present invention relates generally to self-assembled semiconductor quantum dot lasers. More particularly, the present invention is directed towards quantum dot lasers with improved optical gain characteristics.
2. Description of Background Art
Quantum dot lasers are of interest for a variety of applications. Each quantum dot of a quantum dot laser is a three dimensional quantum-confined heterostructure which confines electrons and holes in a region having a size, along each of three dimensions, that is less than a thermal de Broglie wavelength at room temperature (e.g., less than about 100 nanometers in many applications). The quantum confinement produces quantum confined energy states within each dot for electrons and holes. There are also corresponding optical transition energies for both electrons and holes associated with a ground state transition energy, first excited state transition energy, second excited state transition energy, etc. A quantum dot laser typically includes a substantial total number of quantum dots within a gain producing region.
Theoretical studies indicate that quantum dot lasers have many potential performance advantages over conventional quantum well lasers. First, a quantum dot laser has a lower fill factor (volume of material to be pumped) and an improved density of states function compared with a quantum well laser. Referring to FIG. 1, the theoretical density of states function becomes sharper as the carrier dimensionality decreases. FIG. 1A shows a theoretical density of states function for a bulk material, which has a square root dependence on energy. FIG. 1B shows the theoretical density of states function for a quantum well (one dimension of quantum confinement) that increases in steps at each quantum well energy level. FIG. 1C shows the theoretical density of states function for a quantum wire (two dimensions of quantum confinement). FIG. 1D shows the theoretical density of states function for a quantum dot (three dimensions of quantum confinement) that has a delta-like density of states function (e.g., a finite number of states available only at the quantum dot). Theoretical calculations indicate that the threshold current of a semiconductor laser may be improved by using quantum dot active regions due to the smaller volume of material and reduced number of states.
One technique to fabricate a quantum dot laser exploits the tendency of strained layer semiconductors to form islands when the strain exceeds a certain threshold strain. In particular, InAs tends to form islands when a sufficient thickness of InAs is grown on a layer that is psuedomorphically strained on a GaAs substrate due to strain associated with the difference in relaxed lattice constant of the two materials. The InAs islands may be embedded in another layer, such as a GaAs quantum well, to form quantum dots.
Self-assembled quantum dot lasers having a low threshold current density are disclosed by Lester, et. al. in the article entitled xe2x80x9cOptical Characteristics of 1.24-xcexcm InAs Quantum Dot Laser Diodes,xe2x80x9d IEEE Photonics Technology Letters, Vol. 11, No. 8, August (1999). The quantum dot laser structure disclosed in this reference includes a single layer of InAs quantum dots formed by a self-assembled growth process on a GaAs substrate. FIG. 2A shows measured laser spectra for different cavity lengths for the quantum dot lasers of Lester, et al. FIG. 2B shows measured current density versus cavity length for conventional Fabry-Perot lasers having uncoated facets.
Referring to FIG. 2B, the lowest threshold current density operation occurs for a cavity length of almost 3.94 millimeters, for which an emission wavelength of 1.24 microns was achieved. The emission wavelength at 1.24 microns is attributed to a ground state transition energy. When the cavity length is reduced to less than about one millimeter, the lasing wavelength blue-shifts to below 1.15 microns, which is attributed to lasing off of the first excited state. When the cavity length is further reduced to about 500 microns, the emission wavelength is further blue shifted to about 1.05 microns.
One drawback of the quantum dot lasers of Lester, et al., is that the maximum wavelength is shorter than desired, particularly for cavity lengths less than one millimeter. In a variety of commercial applications, a wavelength of greater than 1.260 nanometers (1.260 microns) is desired. This is because a wavelength of at least 1260 nanometers is of interest for use in the OC-48 and OC-192 standard compliant lasers used in short and medium length optical network links. OC-48 and OC-192 are optical carrier (OC) standards for fiber optic networks conforming to the SONET standard. OC-48 has a data rate of 2.4888 Gbps whereas OC-192 has a data rate of 9.952 Gbps.
Another drawback of the quantum dot lasers of Lester, et al. is that the modal gain of the ground state is lower than desired, making it impractical to design short-cavity lasers that lase from the ground state transition energy level. The desired laser length depends upon the application. First, many commercially available packages are designed for a cavity length no greater than 500 microns. Second, directly modulated lasers with conventional facet coatings must typically have a cavity length less than 500 microns, and preferably less than about 300 microns, in order to have a photon lifetime that is sufficiently short to permit high speed direct current modulation. The threshold gain, gth, of the Fabry-Perot lasers is gth=xcex1ixe2x88x92(1/Lcav)1n[R], where Lcav is the length of the cavity, R is the facet reflectivity of both facets, and xcex1i is the internal optical loss. This corresponds to a minimum modal gain of at least 25 cmxe2x88x921 for a typical 500 micron long cavity with uncoated facets and preferably greater than 40 cmxe2x88x921 for a typical 300 micron long cavity with uncoated facets. Unfortunately, the ground state saturated gain reported by Lester, et. al., is less than 9 cmxe2x88x921. Referring to FIG. 2A, it can be seen that the quantum dot laser structure disclosed by Lester, et al. lases at a wavelength of about 1.05 microns for a cavity length of 500 microns or less, associated with an abrupt jump to lasing at higher excited energy states.
A general problem with self-assembled quantum dot fabrication processes is that heretofore the growth processes have not been understood well enough to adjust quantum dot parameters that determine the optical gain characteristics. Consequently, the optical gain spectrum of self-assembled quantum dot lasers may lack sufficient saturated modal gain at a desired wavelength, have a larger than desired separation between the ground state and excited state transition energies, or have either too little or too much inhomogenous broadening.
What is needed are techniques for forming self-assembled quantum dot active regions having desirable optical gain characteristics.
Techniques for forming optical devices having layers of quantum dots having desirable optical gain characteristics are disclosed. In one embodiment, the quantum dots are self-assembled InAs quantum dots formed in InGaAs quantum wells that are grown on a GaAs substrate by molecular beam epitaxy. A first barrier layer of AlGaAs or GaAs is grown. A first well layer of InGaAs is grown on the first barrier layer. A sufficient monolayer equivalent thickness of InAs is grown on the InGaAs to form self-assembled islands. A second well layer of InGaAs is grown over the InAs islands to embed the quantum dots. A second AlGaAs or GaAs barrier layer is then grown to complete the quantum well. The optical gain characteristics of the quantum dot layers are influenced by the compositional uniformity of surrounding layers, the dot size distribution, the dot density, and the number of layers of dots that can be placed in an active region without exceeding a critical thickness for forming dislocations.
In one embodiment, the density of dots is adjusted by selecting the composition of the underlying well material that the dots nucleate on and by selecting the growth temperature of the dots. The growth temperature also influences the size distribution of the dots such that in one embodiment the temperature and equivalent monolayer coverage of InAs for the dots is selected to achieve a desired size distribution of the dots. In one embodiment, the well material has an Indium alloy composition of between about In00.15Ga0.85As to In0.20Ga0.8As. In one embodiment the growth temperature of the dots is selected to be in the range of between about 450xc2x0 C. to 540xc2x0 C.
In one embodiment, the compositional uniformity of the underlying InGaAs is improved by depositing a floating layer of InAs to pre-saturate the InGaAs, thereby permitting an extremely thin bottom well layer to be grown prior to dot formation. The underlying bottom well layer of InGaAs that the dots nucleate may have a thickness of about two nanometers or less, and in one embodiment has a thickness of about one nanometer.
In one embodiment the spatial uniformity of the dots is improved by performing a desorption process to desorb excess InAs from the top InGaAs well layer that is grown over the dots. The desorption step may be carried out at a temperature of between 560xc2x0 C. to 650xc2x0 C. In one embodiment, the desorption process is continued for a sufficient length of time to planarize portions of InAs dots extending above the top InGaAs well layer, thereby improving the uniformity of the InAs dots.
The growth temperature of layers grown subsequent to the quantum dots may be selected to prevent a blue-shift of the emission wavelength of the dots to shorter wavelength. In one embodiment, the growth temperature of cladding layers grown subsequent to the dots is selected to be less than 610xc2x0 C.
In one embodiment, quantum dot lasers are formed having one or more quantum dot layers. In one embodiment, the growth conditions are selected to achieve a size distribution of the dots that produces substantial inhomogenous broadening of the optical gain spectrum, which is beneficial for tunable laser and arrays of lasers having a wide range of operating wavelength. In another embodiment, the layer structure and growth conditions are selected to achieve a saturated ground state modal gain greater than about 25 cmxe2x88x921.